The present invention relates in general to the field of ionizing radiation pulse spectroscopy and more specifically to a device for indicating the correct Pole-Zero Cancellation in Spectroscopy Amplifiers.
It is well known that in ionizing radiation spectroscopy, that is, in measurements of energies of radiations such as alpha, beta and gamma rays, accelerated ions etc., it is usual to employ radiation detectors, preamplifiers, amplifiers and multichannel pulse-amplitude analyzers. The present invention concerns in particular such spectroscopy amplifiers.
As it is also known Spectroscopy Amplifier has to filter the noise and process the pulses received from a preamplifier, associated to the detector, in such a way as to satisfy so that it be possible stringent requirements, so that it be possible to exploit, in the operation conditions, the high resolution obtainable with modern semiconductor detectors. In the mentioned spectroscopy amplifiers the output pulse shape may be different depending on the circuit configuration of the amplifier itself. In practice, the most widely employed and satisfactory pulse shaping is the so-called semi-gaussian shaping, which provides, through active filters, a pulse shape that approximates a gaussian function.
Whatever is the selected pulse shape, it is very important to make sure that, in the actual operating conditions, this shape is not altered by spurious "tails", that is, by additional contributions having low amplitudes, but long durations, extending over time intervals much longer than the envisaged pulse width. In fact, these "tails" deteriorate the resolution of the amplitude measurement in all practical cases where the pulse repetition rate is not very low.
Spurious tails can be caused by an imperfect matching of the amplifier filtering and pulse-shaping networks to the shape of the pulse received from the preamplifier. This pulse has usually a quite short rise time followed by an exponential decay, having a time-constant T.sub.pa, which normally varies from a few tens of microseconds upwards.
Since the value of T.sub.pa varies from preamplifier to preamplifier, all modern spectroscopy amplifiers include control means which makes it possible to adjust the amplifier pulse-shaping in order to match it to the specific preamplifier used, by exploiting the Pole-Zero adjustment technique, thereinafter called P/Z.
For the sake of brevity, in the following the Pole-Zero Adjustment will be denoted simply by P/Z.
A main problem associated with the mentioned spectroscopy amplifiers is to be able, while acting on the P/Z control, to find out the setting at which the pulse has an accurate baseline recovery, at the end of its designed shape. To this aim, it is required to ascertain the existence of slow tails having amplitudes even of the order of 10.sup.-3 of the pulse peak and less, with positive or negative signs. This means to verify at the amplifier output the existence of tails of the order of 10 mV and less, following pulses with amplitudes up to 10 V. This check has to be made in the actual operating conditions, with an electronic noise having an amplitude comparable to or even higher than that of the tails to be checked.
The adjustment is usually made by observing directly on an oscilloscope the waveforms. In such a procedure, besides the difficulty of evaluating the amplitude of tails buried in the noise, other difficulties arise since the oscilloscope is overloaded by the main pulse, which precedes the tail. As a matter of fact, in order to observe the tail, it is necessary to use an expanded vertical scale of the oscilloscope, typically from 10 to 50 mV/div.; in these conditions, a 10 V pulse perturbs the oscilloscope input circuits and deforms the oscilloscope response also in the following time interval, where the tail should be observed. At some extent, this may be avoided by "clipping" the main pulse with a diode network, added at the oscilloscope input (see FIG. 3). However, even with such a diode network, the maximum amplitude of the clipped, pulse may reach 0.5 V, so that it may still overload the oscilloscope on the most expanded vertical scales. The mentioned drawback is thus not eliminated, but only reduced, and therefore limits the maximum sensitivity that can be used in practice for observing the tails.
If, instead of observing artificial pulses produced by a pulse generator connected to the preamplifier test input, the adjustment is made by observing pulses produced by the detected radiations, as in principle it is advisable to do, a further difficulty arises because in general there is a statistical distribution of the amplitudes of such pulses, and therefore of their tails. These fluctuations obviously enhance the difficulty of ascertaining the amplitude of the tails.
In summary, the P/Z adjustment by means of an oscilloscope requires a very skilled operator, and is affected by the following drawbacks:
(a) an oscilloscope must be available; in practical instances this fact may cause difficulties to many of the users. PA1 (b) The noise at the amplifier output obscures the small tails. PA1 (c) The vertical sensitivity which can be employed in the oscilloscope must be limited, in order to avoid that the oscilloscope be overloaded. PA1 (d) The measurement of the radiation pulse tail amplitude is made very difficult by the statistical amplitude distribution of the pulses themselves.